This invention relates to fastening systems and, more particularly, to apparatus for ultrasonically measuring the tensile load in a threaded fastener such as a bolt.
There are a wide variety of fasteners used in wide range of applications. Regardless of the application, however, all involve the use of a fastener to produce a clamping force on two or more components to connect them together. A primary factor in the performance of the connection is whether or not a correct amount of clamping force is used. Too little clamping force results, for example, in slippage or leakage. Too much force causes the connection to ultimately fail.
The accuracy with which a clamping force is applied does depend greatly upon the particular application. If wooden structural members are to be bolted together, for example, an adequate connection is usually achieved simply by applying enough clamping force to imbed the head of a bolt, or its associated washer, in the surface of the wood. On the other hand, where sections of an airframe are bolted together, precise clamping forces must be applied to result in a proper connection. As a practical matter, the importance of accurate tightening control is a direct function of the complexity and performance criteria of the fastener application.
The continued development of lighter, more efficient structures and machines, has mandated better methods of controlling fastener loading. Without discussing the various methods currently in use, suffice it to say that each relies, in one form or another, on the measurement of physical phenomena which is related, in some manner, to a clamping force generated by the tightening of a fastener. Where only one or two connections must be monitored, these various methods are adequate. If, however, there is a large number of connections, and the clamping force of each must be controlled, the relationship between the measured effect of an applied clamping force becomes less predictable. If the method of measurement involves applied torque, this unpredictability increases.
Basically, in fastening systems, torque is that effort required to turn a nut against the inclined plane of the threaded portion of a bolt. While, theoretically, it should be possible to use torque as a measure of fastening force, tests have shown that frictional forces absorb the majority of the tightening forces produced. Consequently, torque turns out to be an unreliable measure, especially when factors such as the material from which the fastener is made, thread fit, and lubrication must be taken into account.